1. Field of the Invention
The present invention relates to a looper drive mechanism for a sewing machine, and more particularly to a looper drive mechanism for transmitting rotation of a main shaft to a looper.
2. Description of the Prior Art
A looper drive mechanism with a grooved cam as shown in FIG. 6 has been conventionally used as a looper drive mechanism for an overlock machine (Japanese Patent Application Laid-Open No. Sho 62-176487). In this looper drive mechanism, a barrel-shaped drum cam 201, provided with grooves 201a and 201b, is attached to a main shaft 200. When the barrel-shaped drum cam 201 rotates together with the main shaft 200, the rollers 203a and 203b engage the grooves 201a and 201b, resulting in oscillation of the looper drive shafts 206 and 207.
However, in the mechanism of this type with the barrel-shaped drum cam 201, since the drum cam 201, the looper drive shafts 206 and 207 and the looper are assembled by attaching to the main shaft 200 during the assembling process, the manufacture process is complicated, time-consuming and laborious. Also, in order to machine the grooves 201a and 201b in the drum cam 201, it is necessary to design and manufacture a special machine, resulting in increased manufacture cost.
To cope with such problems, a looper drive mechanism with a slant cam and a cam follower has been proposed (Japanese Patent Application Laid-Open No. Hei 5-15681). In this looper mechanism, as shown in FIG. 7, looper drive cams 301 and 302, each of which is a cylindrical slant cam are fixed to a main shaft 300. The looper drive bifurcated cam followers 305 and 306, each in the form of a U-shaped cam follower, are attached to looper drive shafts 303 and 304, and are engaged with the looper drive cams 301 and 302. The looper drive bifurcated cam followers 305 and 306 are mounted on the looper drive shafts 303 and 304 with freedom to cancel the displacement between the main shaft 300 and the looper drive shafts 303 and 304. Accordingly, as the main shaft 300 rotates, the slant angle of the cam surfaces of the looper drive cams 301 and 302 varies in the range of .+-..theta. in a plane including the main shaft 300, whereby the looper drive bifurcated cam followers 305 and 306 are angularly moved to rotate the drive shafts 303 and 304.
However, the looper drive cams 301 and 302 and the looper drive bifurcated cam followers 305 and 306 are in line-contact with each other. Accordingly, when the looper drive cams 301 and 302 are angularly moved, the looper drive cams 301 and 302 and the looper drive bifurcated cam followers 305 and 306 generate heat due to frictional resistance. As a result, the durability of the looper drive cams 301 and 302 and the lopper drive bifurcated cam followers 305 and 306 suffers. It is therefore difficult to rotate the main shaft 300 at a high speed. Also, since the frictional resistance adversely affects the components even at a low rotational speed, a large motor having a large output power is required.